TS 1109 
.H76 
Copy 1 



DEPARTMENT OF COMMERCE 



Technologic Papers 

OP THE 

Bureau of Standards 

S. W. STRATTON. Director 



No. 194 

A PRELIMINARY STUDY OF TEARING 

INSTRUMENTS AND TEARING TEST 

METHODS FOR PAPER TESTING 



BY 



PAUL L. HOUSTON, Associate Physicist 
Bureau of Standards 



JULY 27, 1921 




PRICE. S CENTS 
Sold only by the Superintendent of Documents, Government Printing Office 
Washington, D. C. . 

WASHINGTON 
GOVERNMENT PRINTING OFFICE . 
1921 



DEPARTMENT OF COMMERCE 



Technologic Papers 



OF THE 



Bureau of Standards 

S. W. STRATTON, Director 



No. 194 

A PRELIMINARY STUDY OF TEARING 

INSTRUMENTS AND TEARING TEST 

METHODS FOR PAPER TESTING 



BY 



PAUL L. HOUSTON, Associate Physicist 

il 

Bureau of Standards 



JULY 27, 1921 




PRICE, S CENTS 

Sold only by the Superintendent of Documents, Government Printing Office 

Washington, D. C. 

WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1921 



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A PRELIMINARY STUDY OF TEARING INSTRUMENTS 

AND TEARING TEST METHODS FOR 

PAPER TESTING 

By Paul L. Houston 



ABSTRACT 

In this technologic paper a study is made of the relative effect of different sizes of 
test samples on the tearing strength of paper. A great number of samples of commer- 
cial papers are torn on three different instruments, using different sizes of test samples 
and also the same sizes of test samples. Data are collected, accordingly, to show that 
the larger the test sample the greater are the values of tearing strength. The reason for 
this is brought out as fabric assistance, which is of considerable importance in the 
textile industry. 

The three instruments used in this study are a tensile-strength instrument and two 
types of instruments for determining the tearing strength of paper. These two types 
of instruments are called type I and type II. Type I is a recording instrument, while 
type II is a nonrecording instrument. A study is then made of these two types of 
tearing instruments for the purpose of investigating their accuracy and reliability, so 
that the results of this investigation may benefit the paper industry. 

Conclusions are drawn up to show that the type II nonrecording instrument is the 
more reliable of the two and is within a 5 per cent error on the majority of papers. 



CONTENTS Page 

I. Introduction 4 

1. Statement of problem 4 

II. Discussion of test methods 4 

1. Practical tests with different size test samples 4 

2. Practical test with same size test samples 6 

3. Fabric assistance 6 

III. Discussion of a recording instrument 7 

1. Description of instrument 7 

2. Calibration test 7 

3. Performance test 8 

IV. Discussion of a nonrecording instrument 10 

1. Description of instrument 10 

2. Error caused by beam position 10 

3. Error due to impact of water 13 

4. Performance test 16 

V. Conclusions 17 

3 



4 Technologic Papers of the Bureau of Standards 

I. INTRODUCTION 
1. STATEMENT OF PROBLEM 

For some time it has been a recognized fact in the paper industry 
that there is a need for an instrument that will give numerically 
the tearing strength of paper. Considerable work has been done 
along this line at different laboratories. The result has been that 
two types of tearing instruments have been invented and are used 
to some extent. One of these types is a recording instrument, 
giving a curve showing the maximum and minimum tearing 
strength of paper and the results of five initial tears. The other 
type gives merely the maximum tearing strength by weighing or 
measuring the load applied to the paper to tear it. Both types 
have their good and their bad points, which will be brought to 
light in the following study. 

II. DISCUSSION OF TEST METHODS 
1. PRACTICAL TESTS WITH DIFFERENT SIZE TEST SAMPLES 

Before taking up a study of the instruments themselves it must 
be remembered that the tearing strength of paper depends a great 
deal on the size of the test sample. For instance, a series of bag 
papers that had previously been tested for weight (in pounds) of 
the standard size ream 25X40 — 500 and for bursting strength 
were torn on the tensile-strength instrument described in Bureau's 
Circular No. 107, pages 15-17, and on the two types of tearing 
instruments mentioned above. For convenience, let the record- 
ing instrument be called type I and the nonrecording instrument 
type II. The test samples were cut 1 by 8 inches for the tensile- 
strength machine, 2% by \y% inches for type I instrument, and 
4 by 10 inches for type II instrument. All samples were torn the 
machine direction, beginning at a point just halfway across the 
width of the test sample, and an average of 10 tests was taken for 
each result. The results in Table 1 were obtained. It might be 
well to mention here that all tests in this study were conducted 
in a constant-temperature and humidity room, where all test 
samples before they were used were conditioned for two hours 
at a temperature of 70 F and a relative humidity of 65 per cent. 
From these data it is to be noted that the larger the test samples 
are in size the higher are the values of tearing strength. 



Study of Tearing Instruments 



TABLE 1.— Relative Effect of Different Size Test Samples on Tearing Strength of 
Bag Paper, Using Tensile Strength, Type I, and Type II Instruments 

[Test samples: Tensile strength, 1 by 8 Inches; type I, 2 3 /« by l'/e Inches; type II, 4 by 10 Inches] 





Weight 
of the 
standard- 
size 
ream, 
25X«0— 
500 


Bursting 
strength 


Tearing strength 


Bag paper Identification numbers 


Tensile 
strength 


Type I 


Typen 


16 081 


Pounds 
32 
44 
52 
49 
53 

61 
32 
46 
77 

77 

110 
147 
39 
45 
62 

45 
38 
51 
65 
43 

51 
72 
52 

76 


Points 
11 
23 
18 
29 
36 

36 
32 
36 
54 
31 

94 
125 
21 
25 
33 

28 
22 
33 
47 
24 

28 
46 
30 
41 


Grams 
30.8 
58.3 
47.5 
86.6 
91.6 

108.3 
50.0 
100.0 
133.3 
97.5 

270.8 

433.3 

38.3 

43.3 

83.3 

79.1 
101.6 
116.6 
165.6 

78.3 

61.6 

136.6 
73.3 
129.1 


Grams 

15.75 
34.80 
38.40 
58.80 
59.80 

63.40 
33.50 
43.20 
90.60 
55.50 
188.00 
256.50 
22.40 
30.80 
62.80 

52.80 
73.10 
74.60 
108.00 
63.50 

39.50 
116.50 
49.90 
88.50 


Grams 
33.6 


16 082 


76.4 


16 083 


68.4 


16 084 


96.2 


16 085 


113.2 


16 086 


128.6 


16 087 


64.0 


16 088 


118.0 


16 089 


179.2 


16 090 


149.2 


16 091 


338.4 


16 092 


471.4 


16 093.... 


60.6 


16 094 


73.2 


16 095 


135.6 


16 096 


94.6 


16 097... 


84.0 


16 098... . 


121.6 


16 099 


179.2 


16 100 


85.2 


16 101 


83.0 


16 102.. . 


186.6 


16 103 


96.2 


16104 


166.8 







In order to illustrate this point further, a series of writing 
papers that had already been tested for weight (in pounds) of 
the standard size ream 25 X 40 — 500 and for bursting strength 
was torn on the tensile-strength machine and type II nonrecord- 
ing instrument. Test samples were cut 1 by 8 inches for the 
tensile-strength apparatus and 4 by 10 inches for the type II 
instrument, and an average of 10 tests was taken for each result. 
The data in Table 2 show the same results as in Table 1. 

TABLE 2.— Relative Effect of Different Size Test Samples on Tearing Strength of 
Writing Paper, Using Tensile Strength and Type II Instruments 

[Test samples: Tensile strength, 1 by 8 inches; type II, 4 by 10 inches] 



Writing paper identifi- 
cation numbers 



Kind of writing paper 



Weight of 

standard- 
size ream 
25X40— 


Bursting 
strength 


Tearing strength 


Tensile 


Type II 


500 




strength 


Pounds 


Points 


Grams 


Grams 


86 


55 


156.9 


181.7 


46 


19 


32.4 


54.5 


24 


10 


29.0 


34.2 


55 


27 


50.2 


77.8 


44 


19 


28.5 


44.8 


10 


5 


13.8 


16.8 


98 


72 


167.2 


230.4 



Ratio of 

tensile 
strength 

to 
typen 



14 731. 
14 732. 
14 733. 
14 734. 

14 735. 
14 736. 
14 737. 



Bond 

Writing. . 
Copying. . 
Printing . 



....do.... 
Manifold . 
Ledger 



Per cent 

86.4 
59.5 
84.8 
64.5 

63.6 

82.2 
72.6 



6 Technologic Papers of the Bureau of Standards 

2. PRACTICAL TESTS WITH SAME SIZE TEST SAMPLES 

On the other hand, if the test samples are cut the same size, 
very good check results will be obtained. Note the following 
results in Table 3 on the above series of long-fibered bag papers 
when test samples were cut 1 by 8 inches for both tensile-strength 
and type II instruments. An average of 10 tests was taken for 
each result. 

TABLE 3. — Relative Effect of Same Size Test Samples on Tearing Strength of Bag 

Paper, Using Tensile Strength and Type II Instruments 

[All test samples were 1 by 8 inches] 





Weight of 
the stand- 
aid-size 
ream 25X 
40—500 


Bursting 
strength 


Tearing strength 


Ratio of 
tensile 


Bag paper Identification numbers 


Tensile 

strength 


Type II 


strength 

to 
Typell 


16081 


Pounds 
32 

44 
52 
49 
53 

61 
32 

46 
77 
77 

110 
147 
39 
45 
62 

45 
38 
51 
65 
43 

51 
72 
52 

76 


Points 
11 
23 
18 
29 
36 

36 
22 
35 
54 
31 

94 
125 
21 
2S 
33 

28 
22 
33 

47 
24 

28 
46 
30 

41 


Grams 

20.8 
58.3 
47.5 
86.6 
91.6 

108.3 
50.0 
100.0 
133.3 
97.5 

270.8 
433.3 
38.3 
43.3 
83.3 

79.1 
101.6 
116.6 
166.6 

78.3 

61.6 

136.6 
73.3 
129.1 


Grams 

21.0 
58.0 
49.6 
90.2 
93.8 

113.0 
50.0 
92.0 
142.2 
104.8 

271.0 
442.8 
41.2 
44.8 
90.4 

74.2 

96.6 
113.4 
160.0 

70.6 

66.8 
139 6 

74.0 
129.6 


Per cent 
99.0 


16 082 


100.2 


16083 


95.8 


16 084 


96.1 


16 085 


97.7 


16 086 


96.0 


16 087 


100.0 


16 088 


108.6 


16 089 


93.8 


16 090 


93.0 


16 091 


99.7 


16 092 


97.8 


16 093 


93.0 


16 094 


96.6 


16 095 


92.3 


16 096 


106.6 


16 097 


102.6 


16 098 


102.7 


16 099 


104.0 


16100 


110.8 


16 101 


92.3 


16 102 


97.8 


16103 


99.1 


16 104 


99.7 







3. FABRIC ASSISTANCE 

After studying the action of the paper as it was torn on each 
instrument it became evident that each fiber in the path of the 
tear received assistance from all the fibers adjoining as far as the 
edges of the test samples. Also, it was noted that the larger the 
test sample was the greater was the amount of assistance that the 
fibers in the path of the tear received from the adjoining fibers, 
provided, of course, that the beginning of the tear was always the 
same distance from the end of the test sample and just halfway 
across the width of the test sample. In the textile industry this 
effect is called fabric assistance and is of considerable importance. 



Bureau of Standards Technologic Paper No. 194 




Fig. i. — The recording instrument, type I 



Bureau of Standards Technologic Paper No. 194 




Fig. 3. -The nonrccording instrument, type II 



Study of Tearing Instruments 7 

III. DISCUSSION OF A RECORDING INSTRUMENT 

1. DESCRIPTION OF INSTRUMENT 

Since it is found that it is necessary to cut test samples the same 
size in order to compare the tearing strength of papers, it is now 
best to proceed to a study of the instruments themselves. The 
recording instrument, type I, Fig. 1, is the type of instrument 
preferred for laboratory use. However, such an instrument 
should be very delicate, sensitive, and accurate. There should 
be the least possible amount of friction between the chart and 
pen point. The recording arm should move freely, with as little 
friction as possible in the bearing. However, there seems to be no 
instrument of this caliber on the market to-day. The instrument 
under study has the appearance of a delicate, sensitive, and accu- 
rate little machine, but has certain defects which will be discussed 
later. It is composed of a sliding plate on which the chart rests 
and a recording arm which moves on a pin-slot bearing and holds 
a small glass capillary pen with a platinum point. The test 
sample of paper is cut by means of a die in such a way as to give 
five initial tears and a curve showing the maximum and minimum 
tear. Small angular slits in the path of the tear make possible 
these five initial tears, and a slit down the center of one end of 
the sample makes it possible for the starting point of the tear to 
be always the same distance from the edges of the paper. The 
test sample is placed on the instrument so that one half of the 
slitted end is fastened to a pin on the sliding plate, while the other 
half is fastened to a pin on the recording arm. The instrument 
is motor driven, and as the plate slides to one side the paper is 
torn and the recording arm is forced down to register on the chart 
the load in grams necessary to tear the paper. Two different 
weights may be suspended at three different positions on the 
projection of the recording arm as factors or multiples of the 
gram readings on the chart. 

2. CALIBRATION TEST 

Before operating any instrument of this kind it is always 
best to calibrate it to determine its accuracy. .This was done 
by hanging dead- weights on the recording arm. With pen point 
barely touching the chart, there was so much friction in the pin- 
slot bearing that it was almost impossible to calibrate the instru- 
ment at all, as the swinging pen point would stop almost anywhere. 
However, the following correction curves, Fig. 2, were obtained 



8 



Technologic Papers of the Bureau of Standards 



for factors, 5, 10, 20, 50, and 100, but they are not reliable for 
reasons mentioned above. It was impossible to calibrate the in- 
strument for any factor below 5 . 

3. PERFORMANCE TEST 

After attempting' to calibrate this instrument it was thought 
best to tear on it a number of samples of paper, using the different 
weights to represent factors. The same bag papers were used as 
before, and from them three identical sets of test samples (num- 
bered 1, 2, and 3) were cut 2^ inches long and 1 y& inches wide. 
Each test sample was torn in the machine direction, and the 
average of the five initial tears was taken as the tearing strength 
of the paper. In the following data, Table 4, it is to be noted 
that different results were obtained for different factors used. 
The values which are given in this table were directly observed 
and have not been corrected by use of the calibration curves of 
Fig. 2. 

TABLE 4.— Relative Effect of Different Factors on Tearing Strength of Bag Paper, 
Using Three Identical Sets of Test Samples and Type I Instrument 

[All test samples were 2% by l' , Inches) 





Setl 


Set 2 


Set3 


Bag paper Identification numbers 


Factors 


Tearing 

strength 


Factors 


Tearing 
strength 


Factors 


Tearing 
strength 


16081 


2 
5 
5 
5 
5 

10 
5 
S 

20 
20 

20 

50 

5 

5 

20 

5 
10 
10 
20 
10 

10 
20 
10 
20 


Grams 
15.7 
30.0 
29.1 
41.7 
42.3 

64.2 

33.5 
41.6 
90.6 
54.6 

177.2 
256.5 
22.4 
29.6 
62.8 

43.1 
73.1 
74.6 
108.0 
63.5 

39.5 
101.2 
49.9 
96.2 


5 

10 
10 
10 
10 

20 
10 
10 

50 
50 

50 
100 
10 
10 
50 

10 
20 
20 
50 
20 

20 
50 
20 
50 


Grams 
17.8 
34.8 
35.3 
58.8 
59.8 

63.4 
24.5 
43.2 
105.5 
55.5 

188.0 
283.0 
30.8 
30.8 
65.5 

52.8 
61.0 
74.0 
114.0 
37.2 

44.6 
116.5 
41.2 
88.5 


10 
20 
20 

20 
20 

50 
20 
20 
100 
100 

100 


Grams 


16082 




16 083 




16084 




16 085 




16086 




16087 




16088 


38.8 


16089 


16 090 


72.0 
189.0 


16 091 


16092 


16093 


20 
20 
100 

20 
50 


22.2 
36.7 
75.0 

46.2 
76.0 


16094 


16095 


16 096 


16097 


16098 


16099 


100 152.0 


16100 


16 101 


50 
100 

50 
100 


56.0 
146.0 

49.0 
101.0 


16102 


16103 


16104 





Study of Tearing Instruments 




10 15 30 25 30 35 
ACTUAL LOAD IN GRAMS 




20 40 SO 80 100 120 140 160180. 
ACiTJAL LOAD IN ORAMS 

-,.- HT-»r-':!-'"ri:.:T:'"fT T ~~T"'" r ! n — l 



?..::i.:; ; .:'j4--''-i---t-n-f 



5 10 J.5 30 35 30 35 40 45 
ACTUAL LOAD IN GRAMS 



50100 150 300 350 300 35040C 
ACTUAL LOAD IN ORAMS 



SOO 



11^,::,.,. -, 



700 tr 



g6oo| 

9 500 1 

e- "+00 g 



200 
100 



■\v/:.: 



: .; -. !■■■ f\ ^iJKi;:^!^ 



200 300 400 500 60070C 
ACTUAL LOAD IN GRAMS 



Fig. 2.— Diagrams showing correction curves for different factors (recording instrument, 

type I) 
52142°— 21 2 



io Technologic Papers of the Bureau of Standards 

IV. DISCUSSION OF A NONRECORDING INSTRUMENT 
1. DESCRIPTION OF INSTRUMENT 

There will now be taken up a study of the nonrecording instru- 
ment, type II, Fig. 3, which is a simplified instrument adapted for 
mill use. An instrument for mill use should be accurate, at least 
within 5 per cent, should be foolproof and yet so simple of opera- 
tion that paper-mill machine tenders can handle it, and should 
check itself under standard conditions within the variation of the 
strength of paper itself. The instrument under study seems to 
come near being an instrument of this caliber when it is used for 
bag paper or heavy writing paper. It was built to test the tear- 
ing strength of bag paper in the machine direction, since paper 
bags usually tear in this direction. It could be used on writing 
paper to determine the tearing strength in either direction. The 
instrument consists of a beam balancing on a knife-edge (the two 
arms of the beam being equal). One end of the beam holds a 
300-cm 3 glass into which water as the load may be poured from 
a 500-cm 3 burette until the paper tears. At the other end of the 
beam one half of one slitted end of the test sample is clamped, 
while the other half and other end of the test sample are clamped 
against a vertical plate opposite. A special die is used for cutting 
a slit and eyelet hole at each end in the middle of the test sample. 
The cubic centimeter or gram readings are taken from the burette 
at the end of each tearing operation. These readings indicate the 
maximum tearing strength of the paper. Two weights may also 
be used and placed, if necessary, at definite intervals on the glass 
side of the beam. Each weight at each position represents a 
certain load in grams. (It is well to state here that due to the 
facts that the two arms of the beam are equal, and that there 
is very little friction because of the knife-edge, the force at the 
tearing end of the beam is actually equal to the weight of water 

in the glass.) 

2. ERROR CAUSED BY BEAM POSITION 

Before using this instrument extensively for tearing it was 
thought best to study it from the standpoint of physics. For 
instance, it was decided to determine the error due to changes in 
moment of force caused by changes in position of the beam during 
the process of tearing. Also, were there errors caused by the 
force exerted by the falling stream of water ? Let there first be 
made a study of the different moments of force caused by different 
positions of the beam. Note the following diagram of beam, 
Fig. 4. 



Study of Tearing Instruments 



ii 



Let BC represent the beam and AR represent a pointer which 
is fastened at the center A of the beam and at right angles to it, 
and which determines by its position in respect to the graduations 
below, numbered o, i, and 2, whether the beam is balanced (and 
in this case horizontal) or is one or two graduations off balance. 
It is evident that when the beam is one or two graduations off 
balance the moment of applied force decreases as the number 
of graduations off balance of the beam increases. In order to 
find the amount of the error introduced by the changes in moment 
of force caused by changes in position of the beam, the moment 
of the applied force was calculated for three different positions 
of the beam, using 100 g as the weight of water in the glass. 
Referring to the diagram, Fig. 4, let MF and NG represent the two 
positions of the beam BC when it is one and two graduations off 
balance, as indicated by the pointer AR, which at the same time 



^N^- 



// 

// 

// 

/ I 



FlG. 4. — Diagram showing change in effect of weight of water with change in position of 
beam {nonrecording instrument, type II) 

takes the respective position A U and A V. By drawing the dotted 
line US perpendicular to A U at point U and dotted line VT per- 
pendicular to AF at point V, and by extending line AR to inter- 
sect dotted line US at S and dotted line VT at T, right-angled 
triangles A US and AVT are formed. By drawing dotted lines 
GE and FD from points G and F and perpendicular to A C, right- 
angled triangles ADF and AEG are formed. Of these triangles 
mgle D A F = angle SA U, and angle EA G = angle TA V . Distances 
\R, US, VT, and AC have been measured very carefully on the 
yp'e II apparatus, as follows: 

AR = AU = AV= 8.9 cm; 
US= .48 cm; 
VT= .96 cm; 
^£ = 35.56 cm. 



12 Technologic Papers of the Bureau of Standards 

Referring to triangle A US, 

US 
-rjj = tan angle SA U. ( i ) 

Substituting in (i) 0.48 for US and 8.9 for AU, we get: 

^ = tan angle SAU; 
8.9 & 

or, 0.0539 = tan angle SAU. 

Referring to a table of trigonometric functions, it is found that if 
tan angle SA U = 0.0539, 

cos angle SAU = 0.9986. , (2) 

Since angle SAU = angle DAF, by substituting in (2) cos angle 
DAF for cos angle SAU, we get: 

Cos angle DAF = 0.9986. 

Referring to triangle ADF, 

AD 
cos angle DAF = -^ • (3) 

Substituting in (3) 0.9986 for cos angle DAF, we get: 

AD 

-^ = 0.9986; 

or, AD = AF X 0.9986. (4) 

Since AF = AC = 35.56, by substituting in (4) 35.56 for AF, 

Weg6t: AD = 35.56x0.9986; 

or, 4D = 35.5i. 

When the beam is in the position represented by MF and the 
pointer is at the position of graduation, number 1 , the moment of 
force is represented by the formula: 

Moment of force = AD x weight of water. (5) 

Since 100 g were taken as the load in this case, by substituting in 
(5) 100 for weight of water and 35.51 for AD, we get: 

Moment of force = 35.51 X 100 = 3551. 

Referring to diagram, Fig. 4, when the beam is in the position 
represented by NG and the pointer is at the position of graduation, 
number 2, the moment of force is represented by the formula: 

Moment of force =AEx weight of water. (6) 

By using the triangles AVT and AEG and angles TAV and EAG 
the determination of AE is exactly the same as for AD and is 
found to be 35.35. 



Study of Tearing Instruments 13 

Since 100 g were taken as the load in this case, by substituting 
in (6) 100 for weight of water and 35.35 for AE, we get: 

Moment of force = 35.35 X 100 = 3535. 

Referring to the diagram, Fig. 4, when the beam is in the hori- 
zontal position represented by BC and the pointer is at zero 
graduation, the moment of force is represented by the formula : 

Moment of force = AC x weight of water. (7) 

Since AC = 35.56 and since 100 g were taken as the load in this 
case, by substituting in (7) 100 for weight of water and 35.56 for 
AC, we get: 

Moment of force =35.56 x 100 = 3556. 

Consequently, there are the following moments of force at zero 
graduation and at graduations numbered 1 and 2 : 

Moment of force at o = 3556; 
Moment of force at 1 =3551 ; 
Moment of force at 2 =3535. 

From these results it can be seen that the error due to the different 
moments of force caused by different positions of the beam during 
the process of tearing is less than 1 per cent, which is very small. 
Very few testing instruments of greater accuracy than this are 
built. It is well to state here that due to the fact that at the 
tearing end of the beam the pull can not be exactly perpendicular 
to the beam in any position and that the angle of pull will vary 
with the position of the beam, another error exists. However, 
this error is very small, for the reason that the vertical plate X Y 
(Fig. 4) which holds the test specimen of paper is situated so 
close to the end of the beam that there is very little clearance 
between the end of the beam and the vertical plate. Conse- 
quently, due to this fact and due to the fact that the angular 
displacement of the beam during the process of tearing is never 
more than io° because the beam is so long, the angle of pull is 
always very close to the perpendicular. In this connection note 
the angles of pull, JNA, IMA, and HBA (Fig. 4), which are very 
close to right angles. H represents the starting point of tear, 
which is halfway between the end B of the beam in initial 
position and the lower end Y of the test sample. 

3. ERROR DUE TO IMPACT OF WATER. 

Let there now be taken up the second question : Are there errors 
caused by the force exerted by the falling stream of water? In 
practically all cases the paper began to tear at the balancing 



H 



Technologic Papers of the Bureau of Standards 



position of the beam, and the water was shut off immediately by 
turning of the stopcock which closed the outlet from the burette, 
and the reading from the burette was taken. Consequently, it 
was decided to measure the distance from the end of the burette 
to the surface of the water in the glass at different applied volumes, 
as indicated by the graduations on the burette. This was done 
because the different velocities caused by the falling of the water 
from the burette through different distances to the surface of the 
water in the glass were the controlling factors in determining the 
error due to the impact of water. During this operation the 
beam was held firm in balancing position and a piece of aluminum 
attached to a thread was used to make the measurements. The 
following measurements were obtained : 



Volume applied, 
cm" 

25 

75 

"5 



Distance, 
cm 



16. s 

14. 6 

13-0 



Volume applied, 
cm 3 

J75 

225 

275 



Distance, 
cm 

II. 4 

... IO. 2 

8.9 



Then a small piece of aluminum about the size of a 10-cent piece 
was attached by means of very small wires to the end of the beam 
in place of the glass. This was done in such a way that the water 
from the burette fell directly on the aluminum. The small size 
of the aluminum prevented any water from remaining on its 
surface. The forces were then measured experimentally by 
placing small laboratory weights on the other end of the beam 
to balance the force of the water from the burette on the alumi- 
num. The forces were obtained for heads of water in the burette 
at different graudations (on the burette) corresponding to the 
above applied volumes (since the applied volumes are deter- 
mined by the graduations on the burette), and the burette was 
lowered at the end of each experimental operation so that the 
distances between burette and aluminum were the same as the 
above at the respective applied volumes. The same operations 
were repeated continuously until close checks were obtained. 
Ten readings were taken at each head of water and an average 
of the 10 made. The results which were obtained are shown in 
Table 5. (It might be added here that the force of impact on 
the aluminum disk is not exactly the same as the force of impact 
on the water in the glass, because the energy dissipated is not 
the same in both cases. However, the use of the aluminum disk 
would seem to give results that are sufficiently accurate for a 
study of an instrument of this type.) 



Study of Tearing Instruments 
TABLE 5.— Forces at Different Heads 



15 



Readings 


Forces In grams for heads at graduations of — 


25 cm 3 


75cm s 


125 cm 3 


175 om> 


225 cm 3 


275 cm' 


1 


4.80 
4.80 
4.80 
4.85 
4.90 

4.90 
4.90 
4.95 
4.95 
4.95 


4.30 
4.30 
4.40 
4.40 
4.45 

4.45 

4.40 
4.30 
4.40 
4.40 


3.80 
3.80 
3.80 
3.80 
3.80 

3.80 
3.80 
3.80 
3.80 
3.80 


3.50 
3.50 
3.50 
3.50 
3.50 

3.40 
3.40 
3.40 
3.40 
3.50 


3.30 
3.30 
3.20 
3.20 
3.30 

3.20 
3.20 
3.30 
3.20 
3.20 


3.10 


2 


3.10 


3 


3.00 


4 


3.10 


5... 


3.00 


6 


3.00 


7 


3.10 


8 


3.00 


9 . 


3.00 


10.. 


3.10 








4.88 


4.38 


3.80 


3.46 


3.24 


3.05 







From the above results a correction curve was drawn, which is 
presented in Fig. 5. 




35 _50_. 75 100 125 150 175.300 335 250 27S 
VOLUME APPLIED IB 0UBTO CENTIMETERS 

Fig. 5. — Diagram showing positive corrections at different applied volumes as indicated 
by head graduations in Table 5 (nonrecording instrument, type II) 

It can easily be seen from Fig. 5 curve, by dividing the ordi- 
nates by the corresponding abscissas, that the errors caused by 
the force exerted by the falling stream of water are as follows: 



Volume applied, 
cm 3 



Error, 
per cent 



25 x 9-5 2 

75 5-84 

125 3.04 



Volume applied, 
cm 3 



i75- 
225. 

275- 



Error, 
percent 

■ 1-97 
1.44 
1. 10 



Practically all the samples of bag paper that were tested on 
this instrument tore above the 75 g load when the test samples 
were cut 4 by 10 inches, which is the size recommended and speci- 
fied by the inventor. This has previously been presented in 
Table 1. Consequently, the instrument is within the 5 per cent 
error on the majority of these grades of bag paper. For writing 
paper, such as manifold, lightweight printings, and lightweight 
writings, and for all lightweight short-fibered papers, the error is 
greater than 5 per cent, as is shown in Table 2. The term "light- 
weight ' here indicates that the weight in pounds of the standard 



i6 



Technologic Papers of the Bureau of Standards 



size ream 25 X 40 — 500 is less than 50. For practically all writing 
papers that are heavier than 50 pounds, and for practically all 
weights of bonds and ledgers, the error would be less than 5 per 
cent. These errors might be decreased somewhat if the burette 
were lowered nearer the glass than was the case in the above 
experiments. However, the decrease would be very small and 
the results would be comparatively the same. 

4. PERFORMANCE TEST 

After studying this instrument from the standpoint of physics 
a number of samples of paper were torn on the machine in order 
to discover whether the instrument would check itself under 
standard conditions within the variation of the strength of the 
paper itself. Two sets of test samples, numbered 1 and 2, were 
prepared from the same bag papers as were used before and torn 
in the machine direction with very good check results, as are 
shown in Table 6. All test samples were cut 4 by 10 inches, and 
an average of 10 tests was taken for each result. From these 
results it can be seen that it is possible to repeat tests on the 
same grade of bag paper with this instrument and get check aver- 
ages, and that errors due to side pull at the tear and to personal 
errors in stopping the flow of water are relatively very small. 

TABLE 6.— Relative Effect of Use of Two Identical Sets of Test Samples on Tear- 
ing Strength of Bag Paper, Using Type II Instrument 



Bag paper identifi- 
cation numbers 


Tearing strength 


Ratio of 

set 1 to 

set 2 


Bag paper identifi- 
cation numbers 


Tearing strength 


Ratio of 

set 1 to 

set 2 


Setl 


Set 2 


Setl 


Set 2 


16 081 


Grams 
33.6 
76.4 
68.4 
96.2 
112.2 
128.6 

64.0 
118.0 
179.2 
149.2 
338.4 
471.4 


Grams 
32.4 
74.0 
69.0 
96.6 
110.0 
128.6 

64.0 
117.4 
185.6 
147.8 
335.4 
477.0 


Per cent 
103.7 
103.2 
99.2 
99.6 
102.0 
100.0 

100.0 
100.4 

96.6 
101.0 
101.0 

99.8 


16 093 


Grams 

60.6 
73.2 

135.6 
94.6 
84.0 

121.6 

179.2 
85.2 
83.0 

186.6 
96.2 

166.8 


Grams 

56.8 
73.0 

132.4 
92.2 
85.4 

122.0 

176.8 
83.6 
83.2 

186.8 
96.0 

167.8 


Per cent 

106.7 


16 082 


16 094 




16 083 


16 095 


102.2 


16 084 


16 096 


102.6 


16 085 


16 097 . 




16 086 


16 098 . . . 


99.6 


16 087 


15 099 


101.4 


16 088 


16 100 




16 089 


16 101 




16 090 


16 102 .. 




16 091 


16 103 


100.1 


16 092 


16 104 


99.4 









Study of Tearing Instruments 1 7 

V. CONCLUSIONS 

Conclusions to be drawn from this study of type I and type II 
instruments are that neither one of them has been perfected 
enough for general commercial use. Type I, the recording in- 
strument as now manufactured, is neither a delicate, a sensitive, 
nor an accurate piece of apparatus, since the amount of friction in 
the pin-slot bearing and the friction between the pen point and the 
paper chart do not allow careful accurate calibration. Since the 
test results obtained by using different factors will not check, it 
would indicate also that there is a defect in the mechanism of the 
instrument. Type II, the nonrecording instrument, is a fair in- 
strument for bag paper, since most bag papers tear above the 75 
cm 3 or gram mark. However, many recommendations could be 
made, such as stronger clamps, a better device for cutting test 
samples an exact size, a better device to control the distance of 
tear of each sample and to keep the distance the same for all 
samples, and the elimination of the use of weights on the glass side 
of the beam, by which the force is immediately applied instead of 
being gradually applied as in the case of the water. The general 
idea of both instruments is good. The type I recording instru- 
ment gives a curve showing the maximum and minimum tearing 
strength of paper as well as five peaks in the curve showing the 
results of five initial tears. No fault can be found with a curve 
that represents the maximum and minimum tearing strength, and 
the initial tearing strength is just what is wanted for writing 
papers. However, there is great doubt whether the five peaks 
in the curve of type I instrument actually represent five initial 
tears. The fibers very near the edge of an angular slit in the test 
sample (or perhaps halfway between two angular slits between 
which the paper is torn) may be stronger than those fibers at the 
very edge. In such a case the result would be a rising curve and 
the peak would not represent the initial tear but the tearing 
strength of the fibers near the edge or halfway between two slits in 
the paper. During the work on this instrument it was noticed that 
some peaks in the curves were double- toothed or double-peaked. 
In this case the first peak probably more nearly represents the 
initial tear. This may be a very fine distinction, and yet if this 
recording instrument is going to be used as a laboratory instru- 
ment it must be accurate to the highest degree. The type II non- 
recording instrument, on the other hand, gives merely the maxi- 
mum tearing strength of paper, which is all that is necessary for a 



1 8 Technologic Papers of the Bureau of Standards 

mill test. All tests on the two instruments were made in the 
machine direction of the paper (the direction in which paper moves 
on the paper machine) for the reason that better comparative 
results could be obtained in this way. Most papers are much 
stronger in the cross direction than they are in the machine 
direction. Since this is true, if you attempt to tear these papers in 
the cross direction, the direction of the tear as a rule turns to the 
machine direction soon after the beginning of the tear. This 
change in the direction of the tear never occurs when the paper is 
torn in the machine direction. Since in practically all grades of 
paper a good tearing strength in the machine direction is just as 
essential as a good tearing strength in the cross direction, and since 
better comparative results are obtained by tearing paper in the 
machine direction, it would seem that all tearing tests should be 
made in the machine direction. 

Washington, January 5, 1921. 



LIBRARY OF CONGRESS 




